Diamonds have long been used in jewelry due to their long life and aesthetic appeal. Diamond materials also have a range of desirable properties for a large number of different technical applications. For example, diamond material is light in weight and very stiff/rigid. These properties result in diamond being an excellent material for use in forming a speaker dome for high-end audio equipment. Such speaker domes can form high frequency tweeters with a very high break-up frequency beyond the human audio range so as to produce a very high quality sound in the human audio range.
For example, WO2005/101900 discloses such a diamond speaker dome. As described in WO2005/101900, harmonics can extend below the fundamental break-up frequency so it is desirable for the break-up frequency to be well removed from the end of the human audio range to ensure that sound reproduction is not impaired by flexing of the speaker dome at high frequency oscillation. WO2005/101900 describes that a speaker dome having a very high break-up frequency can be provided by a synthetic diamond speaker dome having an integral peripheral skirt of specific dimensions. No details of the specific manufacturing method for fabricating such a speaker dome are recited in the document.
GB2429367 also discloses a diamond speaker dome and describes that such a dome can be fabricated by CVD synthetic diamond material on a convex curved substrate to form a synthetic diamond film thereon followed by separation of the synthetic diamond film from the substrate to yield a diamond speaker dome. No details are given regarding the material to be used as the substrate on which the synthetic diamond material is deposited and no details are given regarding the separation technique used to separate the synthetic diamond film from the substrate to yield the diamond speaker dome.
U.S. Pat. No. 5,556,464 and JP59143498 also disclose diamond speaker domes and describe that such speaker domes can be fabricated by chemical vapour deposition of synthetic diamond material on a convexly curved substrate to form a synthetic diamond film thereon followed by separation of the synthetic diamond film from the substrate to yield a diamond speaker dome. These documents give more detail regarding the fabrication process and describe that the synthetic diamond material is deposited on a convexly curved silicon substrate and that separation of the synthetic diamond film from the substrate to yield the diamond speaker dome is achieved by dissolving the silicon substrate in acid.
The present inventors have utilized the aforementioned silicon substrate-acid dissolution process to manufacture diamond speaker domes and confirmed that such an approach can be used to successfully manufacture diamond speaker domes at high yields without incurring cracking of the relatively delicate, brittle diamond speaker domes during the synthesis and substrate removal steps. As such, this approach provides a viable commercial route to fabrication of diamond speaker domes if synthesis conditions are appropriately controlled. However, a substantial cost in such a process for diamond speaker dome production resides in the cost of suitable convexly curved silicon substrates which are dissolved in acid and thus can only be used once. Furthermore, the actual process of silicon substrate acid dissolution is time consuming, costly, and hazardous. Given the nature of the adhesion of diamond to silicon, the substrate does not permit a release process which leaves the silicon substrate intact for re-use in fabricating further diamond speaker domes. Instead, post growth, silicon is required to be acid digested by, for example, HF/nitric acid.
An additional problem with using silicon as a substrate for CVD diamond growth in a CVD diamond growth process, particularly a microwave activated CVD diamond growth process, is power absorption by the silicon at high temperatures, leading to thermal runaway and fracture. Further still, silicon is readily incorporated into CVD diamond during growth, being particularly visible as the 737 nm Si—V defect. As such, the use of a silicon substrate can detrimentally affect the purity of the CVD diamond product.
In light of the above, the present inventors have recognized that it would be advantageous to seek an alternative to the silicon substrate-acid dissolution process to manufacture diamond speaker domes. In particular, it would be advantageous to provide a method in which a re-usable substrate could be employed for the growth of a diamond speaker dome wherein the substrate is left substantially unaffected by the growth procedure. This would allow substrates to be reused for numerous growth runs and would significantly reduce the associated costs of production. In addition, such a method would also avoid the costly and hazardous method of acid digestion.
The present inventors have thus investigated possible alternative methods and particularly the possibility of using convexly curved refractory metal substrates. In this regard, it is known that planar CVD synthetic diamond films can be grown on planar refractory metal substrates, such as molybdenum, tungsten, niobium, or alloys thereof. For example, U.S. Pat. No. 5,261,959 suggests a refractory metal substrate material such as molybdenum in the form of a planar circular disk. Alternatively, Whitfield et al. suggest the use of a tungsten substrate (see “Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates”, Diamond and Related Materials, Volume 9, Issues 3-6, April-May 2000, Pages 262-268). Specifically, Whitfield et al. disclose the use of a planar polycrystalline tungsten disc 6.3 mm thick and 50 mm in diameter and a single crystal tungsten disc 6.3 mm thick and 8 mm in diameter in a 2.45 GHz microwave plasma reactor. It is taught that substrates are subjected to preparation steps including polishing to a mirror finish with a 1-3 micrometer diamond abrasive and cleaning via ultrasonic washing and an in situ plasma etch. Substrate temperatures are monitored using optical pyrometry and an embedded thermocouple during CVD diamond growth. Spontaneous delamination of the CVD diamond wafer from the tungsten substrate on cooling after growth is also disclosed to yield a free-standing diamond wafer due to the differences in thermal expansion coefficient between the CVD diamond wafer and the tungsten substrate. Whitfield et al. note that generally in their experiments the substrates were not reused but in the few cases where re-use did occur, substrates were lapped and polished for at least 24 hours to remove the thin carbide layer formed during the previous growth run.
In light of the above, it is evident that carbide forming refractory metals may provide an attractive alternative to silicon substrates. Despite this, the present inventors have experienced a number of problems when using such substrates, even in a planar configuration. These include: non-uniform CVD diamond growth over the substrate; delamination of the CVD diamond wafer from the substrate during CVD diamond growth; and crack initiation and propagation during cooling after growth of the CVD diamond wafer. These problems tend to be exacerbated when larger substrates are used for growing large area polycrystalline diamond discs (e.g. 80 mm diameter or more). The problem has been found to be further exacerbated if non-planar substrates are provided such as convexly curved refractory metal substrates. Furthermore, these problems tend to be exacerbated when the substrates are reused in subsequent growth runs. This is particularly problematic as the substrates are expensive and reuse is desirable in an economically competitive industrial process.
Alternatives to the use of a silicon substrate-acid dissolution process to manufacture diamond speaker domes have been proposed in the art. For example, GB2427878 discusses that a diamond speaker dome can be grown on a metallic or non-metallic substrate but identifies potential problems with both approaches. With regard to metallic substrates it is identified that the diamond film grown thereon tends to crack during synthesis or on cooling. With regard to non-metallic substrates it is identified that such substrates are difficult to remove from a diamond film grown thereon. As such, GB2427878 would appear to identify some of the problems which have also been identified by the present inventors as discussed above. In order to solve these problems, GB2427878 suggests that a polycrystalline CVD synthetic diamond speaker dome could be grown on a convexly curved polymer substrate comprising a buffer layer. It is suggested that such a buffer layer may be formed of diamond-like carbon (DLC), amorphous carbon or nano-crystal diamond (NCD), or a metal or ceramic film. It is described that during diamond growth the polymer substrate is thermally decomposed to yield a composite speaker dome comprising a layer of polycrystalline CVD diamond material bonded to a layer of buffer material. Such a method is proposed in to avoid problems of cracking (as when using a solid metal substrate) and to avoid the problems of post-growth substrate removal (as when using a silicon substrate). However, the method described in GB2427878 does not allow for the reuse of substrates as each substrate is thermally decomposed during the diamond growth process. Furthermore, the resultant diamond speaker dome product comprises a layer of buffer material adhered to the diamond dome. Such a buffer layer will tend to detrimentally affect the acoustic properties of the speaker dome, for example by reducing the break-up frequency.
JP4161000 suggests the growth of a polycrystalline CVD diamond speaker dome on a convexly curved tungsten substrate followed by removal of the tungsten by acid dissolution. Such a process is in many respects the same as the previously described silicon substrate-acid dissolution process and possesses the same problems, i.e. the substrates cannot be reused and a costly and hazardous acid digestion step is still required to release the diamond speaker dome from its substrate.
JP60141697 would appear to solve some of the aforementioned problems. This document discloses a method of fabricating diamond speaker domes on a convexly curved substrate made of materials having high heat resistance and low thermal conductivity such as molybdenum and silicon. It is stated that a diamond film can be grown on such a substrate via a CVD technique. After diamond synthesis and cooling, a heating beam of light from an infrared lamp or a heater is irradiated and transmitted through the thin film of diamond to heat the surface of the substrate. It is suggested that the temperature of the substrate is not elevated as much as the diamond film due to the low thermal conductivity of the substrate material and that the diamond film thermally expands due to the temperature difference and is released from the substrate to obtain a free-standing diamond speaker dome.
In principle, it would appear that the method as described in JP60141697 could provide a route to manufacturing diamond speaker domes which allows re-use of substrates and also avoids a costly and hazardous acid digestion step. JP60141697 suggests that the thermally induced release process can be used for both metallic (molybdenum) and silicon substrates. However, as previously indicated, the present inventors have found that given the nature of the adhesion of diamond to silicon, the use of a silicon substrate does not permit a thermally induced release process which leaves the silicon substrate intact for re-use in fabricating further diamond speaker domes. Having regard to the use of metallic substrates such as molybdenum in such a process, as previously indicated, the present inventors have experienced a number of problems when using such metallic substrates including: non-uniform CVD diamond growth over the substrate; delamination of the CVD diamond wafer from the substrate during CVD diamond growth; and crack initiation and propagation during cooling after growth of the CVD diamond wafer. As previously stated, these problems already exist for planar refractory metal substrates and are exacerbated if non-planar substrates are provided such as convexly curved refractory metal substrates. Furthermore, these problems tend to be exacerbated when the substrates are reused in subsequent growth runs. These problems have also been alluded to in GB2427878 which identifies that a diamond film grown on a metal substrate tends to crack during synthesis or on cooling. JP60141697 does not appear to address these problems. Indeed, JP60141697 teaches that the diamond speaker dome remains adhered to the metallic substrate after cooling down from growth conditions and prior to application of a heating beam. The mismatch in thermal expansion coefficient between a diamond film and a metallic substrate on cooling after CVD diamond growth would result in a large amount of thermal stress on the diamond speaker dome due to thermal expansion coefficient mismatch with the underlying substrate to which it is bonded. As such the resultant speaker domes would be likely to contain cracks. Furthermore, JP60141697 still requires a post-growth treatment step to remove the domes which will add time and cost to the manufacturing process.
It is an aim of certain embodiments of the present invention to solve the aforementioned problems. In particular, embodiments of the present invention aim to provide a method of fabricating a diamond speaker dome which permits the controlled delamination of a diamond speaker dome from a reusable substrate without incurring cracking of the diamond speaker dome, without damaging the substrate so that it can be reused, and without requiring post growth treatment steps to achieve delamination.